There are three basic types of rechargeable batteries: nickel cadmium, nickel metal hydride, and lithium ion. Lithium Ion batteries are a high energy density, 3.6V battery. It uses lithium metallic oxide in its positive electrode, or cathode, and carbon material in its negative electrode, or anode. Lithium ions inside the battery transfer between the positive electrode and the negative electrode during charge or discharge.
Rechargeable batteries can be embedded or removable from the device. Many battery chargers use a dual rate charge sequence in which the battery under charge is charged at a fast rate for a period of time, and then charged at a slower or “trickle” rate once the battery has reached a predetermined charge level.
Rapid charge sequences are terminated by using either an inflection in the battery voltage versus time (V), or inflection in the temperature versus time (T), or when the battery reaches a certain voltage constant current constant voltage (CC-CV). At this point, typically the charger switches to a lower rate of charge. This lower rate of charge could be a fixed lower charge rate, or a variable rate, which is lower than the full rate. This rate is called a trickle charge rate or a top-off charge rate. For Lithium Ion batteries, this rate is adjusted so that the battery voltage does not exceed a predetermined voltage value. This is known as the CV portion of the CC-CV profile.
Lithium Ion batteries are typically charged by using the CC-CV method. A CC-CV method algorithm charges the battery at a fixed current rate up to a predetermined voltage. Once the predetermined voltage is achieved, the charger switches to a trickle or slower charge rate. The predetermined voltage is typically selected by the manufacturer. For this voltage there is an associated battery capacity and charge cycle life.
Charge cycle life is defined as the number of full charge cycles that the battery can withstand before its capacity degrades to a specified percentage of its original capacity (typically 80%).
A user who charges his battery infrequently would benefit from a higher predetermined voltage which corresponds to a higher capacity but lower charge cycle life. This is because the end user doesn't use as many charge cycles and would likely charge the battery even less often if he got a higher capacity to use between charges.
A user who charges his battery frequently would benefit from a lower predetermined voltage which corresponds to a lower capacity but higher charge cycle life. This is because the end user uses many charge cycles and his battery would last longer if it could wear out at a slower rate.
Accordingly, an algorithm is needed to determine the voltage to charge the battery up to based upon a user's battery usage pattern, eliminating the need of the manufacturer to select a predetermined voltage.
The various aspects, features and advantages of the present invention will become more fully apparent to those having ordinary skill in the art upon careful consideration of the following Detailed Description of the Invention with the accompanying drawings described below.